1. Field of the Invention
The present invention relates to a dielectric lens and a manufacturing process thereof, and more particularly to a dielectric lens used as an element of an antenna for receiving microwaves for communication and broadcasting and a manufacturing process thereof.
2. Description of Prior Art
A conventional dielectric lens used as an element of an antenna for receiving microwaves of 5 GHz or more is conventionally made of a resin, for example, polypropylene, polyethylene, polystyrene or the like. Ceramic powder, which acts as a foaming agent and as a dielectric constant conditioner is added, and the resin is foamed and molded into a dome. Such a conventional dielectric lens is generally produced by injection molding. However, in producing a thick product by ordinary injection molding, there occur a sink mark on the surface and a lot of voids inside.
Therefore, injection compression molding and structural foaming are recently suggested. Even a thick product produced by injection compression molding does not have defects such as a sink mark and a void, and additionally the entire product can obtain a substantially fixed dielectric constant.
However, injection compression molding requires a mold of a complicated structure and an exclusive molding machine, and thus, the facilities are costly. The structural foaming solves the problem of a sink mark and a void. However, a product produced by structural foaming varies in the expansion ratio and in the dielectric constant from portion to portion, and further, a swirl mark on the surface is caused by bubbles.
In the foaming and molding of the conventional dielectric lens, the surface is solidified to be a radome layer. The radome layer protects the inner foamy body from weathering and reinforces the foamy body. However, if the molded lens is taken out of the mold before the radome layer is formed sufficiently thick, the radome layer will be deflected by the expanding force of the foamy body. On the other hand, if the mold is cooled too suddenly or if the mold cooling time is too long, the radome layer will be formed too thick, which lowers the characteristics as a lens. Further, the long mold cooling time lengthens a molding cycle and lowers the production efficiency.
Further, in order to fabricate the dielectric lens as an element of an antenna, the dielectric lens must be provided with a fitting tab which is to engage with a bracket. Conventionally, insert molding and sandwich molding are used for providing the fitting tab. The insert molding is carried out as follows: a fitting tab, which is made of a high strength resin or a metal, is inserted into a mold; an expandable material is injected into the mold; and thus, on completion of the molding, the fitting tab is fixed on the molded article (dielectric lens). In this method, however, a step of making the fitting tab and a step of inserting the fitting tab into the mold are necessary, which requires more cost and time. In the sandwich molding, a radome layer and a foamy body are made of different resins. The sandwich molding is carried out as follows: a radome layer and a fitting tab are integrally made of a high strength resin by injection molding; and an expandable material is injected into the molded article (radome layer) and becomes a foamy body therein. This method, however, requires two injection cylinders and two kinds of materials.
An object of the present invention is to provide a manufacturing process of a dielectric lens which has no sink marks and no swirl marks on the surface and no voids inside and whose electrical characteristics, such as dielectric constant and Q, are fixed entirely, the manufacturing process requiring a mold of a comparatively simple structure and a low cost.
Another object of the present invention is to provide a manufacturing process of a dielectric lens wherein a radome layer with a desired thickness can be formed without deflection and the molding cycle is short.
A further object of the present invention is to provide a dielectric lens which is provided with a fitting tab by a simple process, the dielectric lens and the fitting tab being made of the same material.
In order to attain the objects above, a dielectric lens manufacturing method according to the present invention has a foam molding step in which an expandable material which is a synthetic resin containing a foaming agent is injected in a cavity of a foaming mold and a pressure is applied, and in the foam molding step, the expandable material is injected up to at least about 80 percent by weight and at least about 100 percent by volume of the capacity of the cavity and is foamed at an expansion ratio of not more than about 1.3.
Any synthetic resin can be used, as long as it can bring out a dielectric constant sufficiently high to serve as a dielectric lens and is proper for injection foam molding. For example, polypropylene, polyethylene, polystyrene, polybutylene terephthalate, ABS resin and the like can be used. It is also possible to use a mixture of such a synthetic resin and dielectric ceramics, glass fiber or the like. As the foaming agent, a conventional agent, such as carbon dioxide azo-dicalvonamide, p, p-oxibenzenesulfonic hydrazide, or the like can be used. Because of the foaming agent, the material injected in the mold has a force against the pressure applied from outside, and accordingly, superficial defects (sink marks and swirl marks) and internal defects (voids) of the molded body can be prevented. The mixing ratio of the foaming agent depends on the desired density of the dielectric lens. However, generally, the foaming agent is added at a ratio within a range from about 0.05 to 3.0 percent by weight of the synthetic resin. If the mixing ratio of the foaming agent is less than about 0.05 percent by weight, the effect of preventing defects will not be sufficiently brought out. If the mixing ratio of the foaming agent is more than about 3.0 percent by weight, although a pressure is applied from outside, the expansion ratio will be over 1.3, and in this case, the molded dielectric lens will be poor in the inductivity and other electric characteristics.
As mentioned, a pressure is applied from outside during the foam molding. The expansion of the material by the foaming agent contained therein is inhibited by the pressure, and thereby, a dense body can be made.
In the method, the expandable material is injected up to at least about 80 percent by weight and at least about 100 percent by volume of the capacity of the cavity. Preferably, the expandable material is injected up to a percent within a range from about 85 to 91 percent by weight of the capacity of the cavity. If the expandable material is injected in an amount over 91 percent by weight of the capacity, a burr occurs, and a defective product will be made. If the expandable material is injected up to a percent less than about 85 percent by weight of the capacity, the molded body will be too low in the dielectric constant to have a sufficient antenna gain. The expandable material is foamed at an expansion ratio of not more than about 1.3. Preferably, the expansion ratio is within a range from about 1.00 to 1.17. If the expansion ratio is over 1.17, the molded body is likely to be too low in the dielectric constant to have a sufficient antenna gain. If the expansion ratio is less than 1.0, the molded body is likely to have superficial defects and internal defects.
Weight and volume are interrelated. Thus, the amount of material which fills the capacity (volume) of the cavity can be calculated by multiplying the volume of the cavity by the specific gravity and the result is, of course, expressed in terms of weight. The weight of material injected is preferably less than the weight which would fill the cavity if the expandable material was under ambient pressure.
Another dielectric manufacturing method according to the present invention has a foam-molding step in which an expandable material whose main constituent is a synthetic resin is injected into a cavity of a foaming mold to obtain a dome body with a thin radome layer on the surface, and a shaping step in which the foam-molded body is taken out of the foaming mold and placed in a cavity of a shaping mold which is identical in shape with the foam-molded body. The expandable material, which is in a melted state, is injected into the cavity of the foaming mold and immediately starts foaming, and a radome layer is formed on the surface. When the radome layer becomes lightly solid, the foam-molded body is transferred from the foaming mold to the shaping mold.
In the method, since the foam-molded body is taken out of the foaming mold while the solidification of the radome layer is still light, the foam-molding cycle takes only a short time, and the foaming mold can be used efficiently. The foam-molded body still continues foaming in the shaping mold. However, since the foam-molded body is provided with a proper pressure inside the cavity of the shaping mold, the foam-molded body is not deflected. Also, the radome layer does not grow in the shaping mold any more, and the radome layer is completely solidified to be about 10 mm or less in thickness, which will never degrade the characteristics as a lens.
A dielectric lens according to the present invention is produced as a dome with a radome layer on the surface by injection foam molding of an expandable material whose main constituent is a synthetic resin, and has a fitting tab which is integral with the radome layer and extends outward from the radome layer. The expandable material, which is in a melted state, is injected into a cavity of a mold and immediately starts foaming, and a radome layer is formed on the surface. The cavity of the mold has a recess, and the expandable material deposited in the recess is solidified to be the fitting tab. In this method, a fitting tab can be formed to extend from the radome layer simultaneously with the molding of the dielectric lens. In this method, a fitting tab producing step and an insert molding step can be eliminated. Also, it is not necessary to use two kinds of materials for molding. Thus, a dielectric lens with a fitting tab can be produced in a simple process at a low cost.